Mark asked whether street trees have economic development benefits, on top of the obvious quality of life benefits, and Teresa pointed to a round-up by the Arbor Day Foundation; I decided to go a little further and dig up some primary sources. Copied over here for my own future reference, plus just a little bit of math based on the first source:

$50k median taxable value in Ypsilanti * 3% increase from street trees * 33.67 mills for all local property taxes = $50.51 / year tax revenue increase due to each street tree added to an Ypsilanti home that doesn't have one.

Donovan and Butry (2010) (pdf) used hedonic price modeling on a sample of 3,500 home sales in Portland to find that street trees added an average of $8,870 to the sale price of homes they fronted, and additional value to homes within 100 feet. (About a 3% difference in mean sale price.)

Donovan and Butry (2011) (pdf) then applied a hedonic model to a sample of 1000 single-family rental homes to find that street trees added an average of $21/month to going rental rates (1.6% of the mean rental rate)

Wolf (2005) (pdf) found that if a small city's "main street" business district had a streetscape including trees, customers were willing to travel from further away, stay longer, visit more frequently, and pay more for parking than in a similar business district without trees. (The study also notes that customers are willing to spend 9%-12% more for goods in a shopping district with trees, though that finding was based on districts in larger cities, population 250k+, and apparently was not tested for the "small city", population 10k-20k, category.)

There's a ton of work on the benefits of urban trees in reducing the load on stormwater infrastructure -- and therefore the costs of providing it. However, since Ypsi generally already has all of its stormwater infrastructure, these direct fiscal benefits are likely less significant here.

McPherson and Muchnick (2005) (pdf) use pavement condition and maintenance records from Modesto, CA, to suggest that a strong street tree canopy can reduce street maintenance costs by up to 58% by reducing the deterioration of pavement from sun/heat. (Results might be less significant around here, where we get a little less sunshine than Modesto...)

This 2006 piece by Dan Burden (hosted on Michigan DNR's website) (pdf), proposes that, "For a planting cost of $250-600 (includes first 3 years of maintenance) a single street tree returns over $90,000 of direct benefits (not including aesthetic, social and natural) in the lifetime of the tree" and then summarizes various benefits. It is not a "scientific" piece like the others I've noted, and does not specifically cite its sources--it was by googling bits out of this that I found the actual Wolf and McPherson papers, though, so at least those findings have some backing.